Leakage of secret information has become a serious problem with the progression of social informatization, and the importance of technology for preventing information leakage has been increasing. As the information leakage technology, for example, a technique of encrypting digital data without allowing a third party to learn the contents of the data even if the third party obtains the data was developed. This encryption technique is already used as effective means for preventing the information leakage of digital data.
In the meantime, technology for preventing the information leakage of a printed matter printed on a paper medium, etc. has not been sufficiently established, and has not been commercialized yet. Statistics proves that approximately one half of information leakage cases relate to printed matters in the contemporary society. Accordingly, there is an urgent need to develop technology for preventing the information leakage of a printed matter, similar to digital data.
Specific examples of printed matters, for which countermeasures against information leakage are desired to be taken, include a commodity purchase bill, a statement of a credit card, etc., a medical record of a hospital, a school report card, a list of names, etc. According to JP/2007/000215 (hereinafter referred to as Patent Document 1) filed as a PCT application, an image printed on paper as well as a digital image can be encrypted to prevent information leakage. A hospital medical record, a full statement, etc., which are printed on paper, can be defined as one type of visual information. Accordingly such items of information are collectively referred to as “images” in this specification.
Image encryption disclosed by Patent Document 1 is summarized.
Part of an input image 10 illustrated in FIG. 1(A) is specified as an encryption region 11, and an image of the encryption region 11 is encrypted with an encryption key. As a result, an encrypted image 20 illustrated in FIG. 1(B) is generated. In the encrypted image 20, a region 21 corresponding to the encryption region 11 is encrypted to be visually unreadable.
In the above described input image encryption, the encryption process itself is executed as a digital data process. Therefore, an encrypted image is digital data. This encrypted image may be thereafter printed on a paper medium, etc., or may be exchanged unchanged as digital data. The invention disclosed by Patent Document 1 is characterized in that an encrypted image once printed and converted into so-called analog digital data can be again decrypted.
The invention disclosed by Patent Document 1 adopts a common key (the same key) for image encryption and decryption. Therefore, a key to decrypt encrypted information must be separately transmitted to the transmission destination of the encrypted information. Accordingly, a third party can possibly steal the decryption key, leading to a problem in terms of security.
In the meantime, an electronic authentication system where a robust authentication server is constructed and used to safely exchange a common key has been already commercialized. However, the configuration of a system using such an authentication server is significantly different from that of a popularized simple authentication system intended for individual users. Therefore, users that can benefit from the technique of the invention disclosed by Patent Document 1 are restricted.    Patent Document 1: PCT/JP2007/000215